A magnetic recording medium is widely used as an audio tape, a video tape or a floppy disk. Certain magnetic recording media are fundamentally composed of a non-magnetic support having thereon a magnetic layer containing ferromagnetic particles dispersed in a binder.
High performance characteristics such as electromagnetic characteristics, running durability or running property are required for a magnetic recording medium. That is, higher ability for reproducing original sound is required for an audio tape for recording and reproducing music. Excellent electromagnetic characteristics are required for a video tape to ensure ability for reproducing original images.
It is known that the electromagnetic characteristics of a magnetic recording medium comprising ferromagnetic particles remarkably vary depending upon the dispersion state of the ferromagnetic particles in the magnetic layer. That is, even though ferromagnetic particles having excellent magnetic properties are used to improve electromagnetic characteristics, the excellent magnetic properties do not yield an improvement in electromagnetic characteristics if the dispersion state thereof is poor.
One approach to improving the dispersibility of ferromagnetic particles in a magnetic layer is long term mixing, kneading and dispersing in preparing the magnetic coating composition used for a magnetic layer. However, there is the problem of decrease the magnetic properties of ferromagnetic particles if mixing, kneading and dispersing is conducted for too long a term.
Recently, it was proposed that a polar group should be introduced into the resin component used to form the binder so that the binder in the magnetic layer has good affinity with the ferromagnetic particles.
For example, it is disclosed in JP-A-59-5424 that resins having a predetermined polar group such as a metal sulfonate group should be used in the binder in an amount of 50 wt % or more. The term "JP-A" as used herein means"an unexamined published Japanese patent application". As disclosed above, good dispersibility of ferromagnetic metal particles in a magnetic layer can be secured using resins having a polar group as a binder, and accordingly, electromagnetic characteristics of a magnetic recording medium using such a magnetic layer can be improved
However, a magnetic layer using a polar group containing resin as a binder tends to be hard. Such a hard magnetic layer is likely to adhere incompletely to the support, and the smoothness of the magnetic layer tends to be deteriorated due to poor moldability during calendering.
On the other hand, in view of the molecular weight of resins used as binders, it is known that among resins having the above described polar groups, resins having a lower molecular weight are preferred, because dispersibility of ferromagnetic metal particles is better and electromagnetic characteristics are improved. However, when resins having a low molecular weight are used as a binder in a magnetic layer, the layer tends to be fragile. A fragile magnetic layer has insufficient strength and surface hardness, and tends to have poor running properties and durability.
Accordingly, even though electromagnetic characteristics are fairly good, durability is not satisfactorily excellent.